Swap old bulbs with compact florescent lightbulbs.

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Compact Florescent Lightbulbs
Lighten Up
About a quarter of the energy needed for our homes goes toward keeping on the lights. According to the Energy Star folks, if every U.S. household replaced one traditional lightbulb with an energy-efficient compact fluorescent lamp (CFL), we would save $600 million in utility bills and enough energy to light 3 million homes each year. CFLs cost more, but they use nearly 75 percent less energy and last about 10 times longer than incandescent lightbulbs. That adds up to about $30 of savings over the life of each bulb.